Sustainable Performance Enhancement of Secondary Cold Recycled Asphalt Mixtures with Foamed Asphalt: Gradation, Modified Binder, and Compaction

Cold recycled asphalt pavement technology can achieve the reutilization of aged pavement materials in-place without heating processes, offering cost-effective rehabilitation with ecological benefits for pavement sub-surface layer. However, cold recycled asphalt mixtures often prematurely fail to achieve design service life, developing irreversible damage under service conditions. Consequently, appropriate maintenance or further cold recycling are required to restore pavement sustainability. First, the milled initial cold recycled pavement (secondary RAP) was differentiated from the original aged pavement materials (initial RAP) used during the initial recycling phase. Based on the characteristics of the secondary RAP, the mix design was optimized by incorporating high-elastic modified asphalt (H-asphalt) and employing vibration compaction. The enhanced secondary cold recycled mixture with foamed asphalt (SCRMFA) was then evaluated against the initial cold recycled mixture (ICRM). The decrease in crushing resistance and abrasion resistance indicated that secondary RAP undergoes more pronounced degradation than the initial RAP, highlighting the necessity for gradation optimization. Introducing both coarse and fine aggregates effectively improved SCRMFA performance, the indirect tensile strength (ITS) of the SCRMFA exhibited a significant increase of 0.217 MPa compared to that of the initial RAP. Structural analysis and microstructural observations further demonstrated the synergistic benefits of H-asphalt and vibration compaction, enabling SCRMFA to attain performance levels comparable to those of ICRM. In addition, a 45.6% reduction in life cycle energy consumption and a substantial decrease in carbon emissions were achieved by the implementation of secondary cold recycling, underscoring its environmental advantages and potential for sustainable pavement engineering. This work presents a viable strategy for enhancing pavement performance in secondary cold recycling with foamed asphalt, contributing to the sustainable development of cold recycled mixture.